The present invention relates generally to electromotive devices or electromechanical machines, such as, electric generators or electric motors. More particularly, the present invention relates to a rotor design for an electric motor or an electric generator which utilizes a superconducting coil.
Electric motor and generator designs can utilize coils comprised of superconductors, such as, high-temperature superconductors (HTS). For example, a conventional superconducting synchronous motor construction can have a stator with multi-phase windings (which can have four poles) and a rotor with four superconducting winding assemblies. The use of superconductors in the winding assemblies allows the motor/generator to obtain superior specific power and increased efficiency. The term motor/generator in this application refers to an electromechanical machine or electromotive device which is a generator, a motor, or both a generator and motor.
The superconducting windings in the rotor significantly reduce winding losses, eddy current losses, and hysteresis losses associated with a conventional motor/generator. For example, superconducting motor constructions may be able to achieve efficiencies exceeding 98% in intermediate size motors. Such motors are particularly useful in applications where smaller size, lighter weight and higher efficiency are important. Motors of this type may be very useful in propulsion system applications where low speed is desirable (e.g., very low speed motors operating at speeds of 120 revolutions per minute (rpm)).
One type of superconducting motor/generator is a high temperature superconducting (HTS) motor/generator. Synchronous, HTS electric motors can be designed to have approximately less than half the volume and half the loss of conventional induction or synchronous, non-superconducting motors.
The magnetic circuits of HTS motors/generators are generally designed and constructed without ferromagnetic rotor assemblies. According to conventional designs, the high strength magnetic fields (large magnetic flux density) would fully saturate a ferromagnetic rotor assembly. Therefore, a ferromagnetic rotor/stator assembly (e.g., core) may not positively affect winding working conditions. In addition, ferromagnetic rotor assemblies, such as, magnetic carbon steel cores can become brittle and magnetic losses increase at the low temperatures associated with superconducting coils (e.g., cryogenic, below 77 K). Accordingly, the use of ferromagnetic rotor assemblies can be problematic in high speed HTS motors/generators.
Conventional superconducting wires or tapes utilized in the coil of the rotor of the motor/generator have a critical current density. Critical current density is dependent upon temperature and upon characteristics of the magnetic field. The largest working current density in the superconducting wire or tape must be smaller than the critical current density.
One characteristic of the magnetic field, the flux density perpendicular to the broad surface of the superconducting tape, has a detrimental effect on the critical current density. Generally, to alleviate this detrimental effect, the form of the cross section of the coil of the superconducting tapes is chosen to make the magnetic flux density perpendicular to the tape surface as small as possible. However, this design criteria can limit the form of the cross section of the tape to be within narrow parameters. This design criteria is particularly stringent in the design of low speed electromotive devices which have a large number of poles.
Relying solely on the superconducting coil (eg., winding) to generate the excitation field requires large current densities. Large current densities in the coil can cause additional direct current losses in the superconducting coil. The large current densities can increase the losses associated with the refrigeration system (e.g., the hot side of the refrigerator) and decrease the efficiency of the motor/generator.
Thus, there is a need for a more efficient superconducting electromechanical machine, such as, a generator or a motor. Further, there is a need for a rotor design which is not subject to disadvantages associated with magnetic flux density perpendicular to the tape surface. Further still, there is a need for an improved design of an HTS motor/generator. Even further, there is a need to decrease the volume of an HTS motor/generator. Even further still, there is a need for an HTS motor/generator having an increased working temperature and decreased losses in the refrigerator. Yet further, there is a need to increase the autonomy of a motor/generator.
An exemplary embodiment relates to an electromotive device comprising a stator and a rotor. The rotor includes at least one superconducting winding and at least one permanent magnet.
Another exemplary embodiment relates to a rotor assembly for a synchronous electromotive device or electromechanical machine. The rotor assembly includes a superconducting coil and a permanent magnet.
Yet another exemplary embodiment relates to a method of manufacturing a motor. The method includes providing a stator and providing a rotor. The stator has a number of poles, and the rotor has a number of permanent magnets and a superconducting coil. The magnets and coil are attached to a rotor body. The permanent magnets can be magnetized after the superconducting coil and the permanent magnets are attached to the rotor body. The permanent magnets can also be disposed to reduce perpendicular flux density through the superconducting coil. The rotor body can be a magnetic or non-magnetic material.
Yet another exemplary embodiment relates to a motor/generator architecture. The motor/generator architecture includes a rotor. The rotor has a superconducting coil and a permanent magnet.